Abstract Castration-resistant prostate cancer (CRPC), a fatal disease, remains therapeutically underserved due to limited understanding of molecular factors underlying its emergence and progression. Through unbiased transcriptomics analysis of an early CRPC model developed in our lab and patient dataset analyses, we have discovered that stimulation of the nitric oxide receptor complex, soluble guanylate cyclase (sGC), in conjunction with standard-of-care androgen deprivation (AD) is likely to be therapeutically beneficial in CRPC. Based on our preliminary results, our hypothesis is that decreased sGC activity promotes CRPC growth and that increasing sGC activity will limit CRPC emergence and progression. The rationale of our studies, supported by our findings and well-characterized sGC regulatory mechanisms from the cardiopulmonary field, is that sGC is oxidized and refractory to stimulation in CRPC but is functionally regenerated by AD-induced redox-protective responses. Thus, the combinatorial use of sGC agonists and AD is predicted to be an effective strategy to combat CRPC. The sGC agonist, riociguat, is FDA-approved as a vasodilator in pulmonary hypertension treatment. Our preliminary data show riociguat reduces xenograft CRPC growth. This anti-tumor response is accompanied by lowered PSA levels as well as increased systemic and intratumoral cyclic GMP (cGMP) levels, indicating on- target stimulation of sGC bioactivity. Consistent with the physiologic function of enhanced sGC activity, we find riociguat treatment leads to marked tumor oxygenation and decreased levels of CD44, a key PC stem cell marker. Thus, riociguat efficacy may derive from eradication of hypoxic niches and the residing androgen- refractory stem cell populations thought to underlie CRPC. Hypoxia also alters the tumor redox state through mitochondrially-generated ROS that regulate angiogenic signaling. Through its putative targeting of cancer stem cells and redox vulnerabilities, riociguat is novel in PC therapy, and is safe and well-tolerated longterm. Our objective is to comprehensively establish mechanisms underlying how and why stimulating the sGC pathway limits CRPC growth and progression. Therefore, in these studies, we will assess 1) how mechanisms that control sGC expression and regenerate oxidized inactive sGC are altered in hormone-sensitive vs. castration-resistant cells, 2) how enhancement of sGC bioactivity induces anti-CRPC outcomes through consideration of hypoxia- associated metabolic and redox stresses, including those induced by androgen receptor re-activation under AD, and 3) test riociguat efficacy in the spectrum of CRPC disease, using robust preclinical and patient-derived models encompassing emergence, growth, progression and metastatic bone colonization. We will validate key molecular findings in de-identified PC patient specimens. Our studies possess strong potential to uncover novel mechanisms underlying PC progression, and for clinical translation via repurposing riociguat to treat CRPC.